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1506I

4.5A, 500kHz Step-Down Switching Regulator

Linear Technology 

LT1506

APPLICATIONS INFORMATION

40

GAIN

20

40

VIN = 10V

VOUT = 5V

IOUT = 2A

0

0

–40

PHASE

–20

–80

–40

10

100

1k

10k 100k

FREQUENCY (Hz)

–120

1M

1505 F10

Figure 10. Response from VC Pin to Output

Error amplifier transconductance phase and gain are shown

in Figure 11. The error amplifier can be modeled as a

transconductance of 2000µMho, with an output imped-

ance of 200kΩ in parallel with 12pF. In all practical

applications, the compensation network from VC pin to

ground has a much lower impedance than the output

impedance of the amplifier at frequencies above 500Hz.

This means that the error amplifier characteristics them-

selves do not contribute excess phase shift to the loop, and

the phase/gain characteristics of the error amplifier sec-

tion are completely controlled by the external compensa-

tion network.

In Figure 12, full loop phase/gain characteristics are

shown with a compensation capacitor of 1.5nF, giving the

error amplifier a pole at 530Hz, with phase rolling off to 90°

and staying there. The overall loop has a gain of 74dB at

low frequency, rolling off to unity-gain at 100kHz. Phase

shows a two-pole characteristic until the ESR of the output

capacitor brings it back above 10kHz. Phase margin is

about 60° at unity-gain.

Analog experts will note that around 4.4kHz, phase dips

very close to the zero phase margin line. This is typical of

switching regulators, especially those that operate over a

wide range of loads. This region of low phase is not a

problem as long as it does not occur near unity-gain. In

practice, the variability of output capacitor ESR tends to

dominate all other effects with respect to loop response.

Variations in ESR will cause unity-gain to move around,

but at the same time phase moves with it so that adequate

phase margin is maintained over a very wide range of ESR

(≥ ±3:1).

3000

200

PHASE

2500

150

2000

GAIN

( ) 1500 VFB 2 × 10–3

ROUT

200k

100

VC

COUT

12pF

50

1000 ERROR AMPLIFIER EQUIVALENT CIRCUIT 0

RLOAD = 50Ω

500

100

1k

10k

100k

FREQUENCY (Hz)

–50

1M 10M

1506 F11

Figure 11. Error Amplifier Gain and Phase

80

200

GAIN

60

150

40

100

PHASE

20

50

0

VIN = 10V

VOUT = 5V, IOUT = 2A

COUT = 100µF, 10V, AVX TPS

CC = 1.5nF, RC = 0, L = 10µH

–20

10

100

1k

10k

FREQUENCY (Hz)

0

–50

100k 1M

1505 F12

Figure 12. Overall Loop Characteristics

What About a Resistor in the Compensation Network?

It is common practice in switching regulator design to add

a “zero” to the error amplifier compensation to increase

loop phase margin. This zero is created in the external

network in the form of a resistor (RC) in series with the

compensation capacitor. Increasing the size of this resis-

tor generally creates better and better loop stability, but

there are two limitations on its value. First, the combina-

tion of output capacitor ESR and a large value for RC may

cause loop gain to stop rolling off altogether, creating a

gain margin problem. An approximate formula for RC

where gain margin falls to zero is:

( ) ( )( )( )( ) RC Loop Gain = 1 =

GMP

VOUT

GMA ESR

2.42

19

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